We've acted on our threat to remove the Browser Upgrade Campaign message as it previously existed on this site. Our changes were simple, but we think they make a big difference.
We changed the message to read: "You are viewing the low-tech version of dionidium.com. All site content is accessible to any device."
We feel this message gets right to the point without eliciting the type of negative response a "you-need-to-upgrade" message might.
The second point is the most important, especially for users of mobile devices with small screens. We feel that it's important to have an upgrade message, but not important enough to make it the first thing users see on every page. Note that this approach does little to explain why the user is viewing a low-tech edition, something users may want to know. We recommend linking to a separate document explaining the message, if you feel it's appropriate for your audience.
Unrelated:
Seamus Leahy's CSS2 menu is quite nice, even if it is entirely unusable because -- surprise! -- it doesn't work in IE. (via mezzoblue)